Refrigerated containers are commonly used for shipping perishable cargo, such as fresh produce and other foods, which must be maintained during transit at a temperature within a specified temperature range, thus temperature controlled, to maintain freshness and minimize spoilage. By separating the atmosphere outside the container from the atmosphere inside the container, an enclosed airspace that may be temperature controlled, and thus conditioned, is created. In addition to temperature, the relative humidity, oxygen content, and other parameters may be controlled in the enclosed airspace. Refrigerated containers of this type may be designed to accommodate transport by road on trailers, by sea on container ships, by rail on flat-bed train cars, and by air in cargo planes. While the container may be detachable from the transport vehicle, the container may be fixed to a truck chassis or to a semi-trailer chassis when the transport vehicle is dedicated to transporting refrigerated fresh produce and food. Such dedicated produce and other food transport vehicles are often referred to as reefer trucks.
Refrigerated containers are equipped with a refrigeration unit, commonly referred to as a reefer, which is secured to one wall of the container or to the transport vehicle. The refrigeration unit includes a compressor, a compressor motor, a condenser unit isolated from the enclosed airspace, and an evaporator unit in airflow communication with the interior of the enclosed airspace. In this way, the refrigeration unit can transfer heat from the conditioned airspace enclosed by the container to the atmosphere surrounding the exterior of the container. The refrigeration unit receives electrical power from an external source, such as the transport vehicle or a dedicated generator integrated with the container.
Produce and other foods spoil due to microorganism growth and the generation of spoilage gases, such as ethylene, which increase the rate of spoilage. While light and dehydration also can cause spoilage, when the produce and other foods are transported in an enclosed airspace, microorganism growth and spoilage gases are the primary factors. Microorganisms may include bacteria, viruses, fungus, and mold.
Although not commonly considered, food is refrigerated to substantially slow, not to substantially eliminate, microorganism growth. While slowing microorganism growth through refrigeration is effective, it would be preferable to kill the microorganisms as opposed to only slowing their growth. Thus, refrigeration may be supplemented with sterilants that focus on killing as opposed to slowing the growth of the undesirable microorganisms.
While chemical sterilants have been used to prolong the storage life of produce and other foods, chemical sterilants create the problem of having to remove the sterilant before the produce or other foods are eaten. Many types of food cannot be cleaned of the chemical sterilant before consumption, thus preventing the use of chemical sterilants for these food types.
One way to prolong the storage life of produce and other foods without using chemical sterilants is through the use of ozone, an allotrope of oxygen. Ozone has the ability to directly oxidize harmful bacteria, mold, and spoilage gasses, such as ethylene, thus rendering the bacteria, mold, and spoilage gasses inert from a produce and other food spoilage perspective. Generally, ozone concentrations of 0.1 ppm in an enclosed airspace will destroy microorganisms and eliminate most odors within 48 hours. Although conventional systems have attempted to use ozone as an effective sterilant for produce and other foods, there have been multiple disadvantages.
One disadvantage of conventional produce and other food ozone sterilization systems has been a difficulty in successfully retrofitting existing refrigerated containers with the ozone sterilization system, often arising from the variability in enclosed airspace volumes and the variability in produce and other food types being transported. Another disadvantage has been the inability of these conventional systems to provide enough ozone to effectively control microorganism growth due to an inability to generate and maintain the needed levels of ozone within the container before the container is loaded with produce or other foods before and during transport. Another disadvantage of conventional produce and other food ozone sterilization systems is their relatively high failure rate in actual use.
As seen from the above description, there is an ongoing need for improved devices and methods for at least partially sterilizing produce and other foods before and during transport to extend post-transport shelf life and freshness. The devices, methods, and systems of the present invention overcome at least one of the disadvantages associated with conventional refrigeration-only or refrigeration plus ozone produce and other food transport devices and methods.